1. Field of the Invention
The invention relates to forming connections between electronic parts, such as IC chips, etc., and substrates. More particularly, the invention relates to the arrangement of electrically conductive particles on electrodes of a substrate or electrodes of an electronic part to create an electrical contact.
2. Description of Related Art
Wire bonding and wireless bonding techniques are used to mount electronic parts, such as IC-chips, to substrates, such as ceramic substrates.
More recently, bare-chip mounting methods which fix a chip directly to a substrate have become the focus of attention in the technical field of mounting semiconductor chips. In the bare-chip mounting method, conductive particles are arranged on the electrodes of an electronic part or on the electrode pattern formed on the substrate. Electrodes of the electronic part electrically contact the electrodes of the substrate via the conductive particles.
Japanese Laid-Open Patent Publication No.63-47943/1988 describes a method for affixing coated particles to a substrate to form an electrical connection. An adhesive is sprayed onto an electrode pattern area on a substrate. Globular conductive particles, coated with isolating resin, called microcapsules, are put on the substrate. Some of the microcapsules adhere to the electrodes. The extra microcapsule, which have not adhered to the electrodes, are removed. Then, the electronic part is positioned relative to the substrate, and the electronic part and substrate are treated by heat and pressure to form an electrical connection.
This method, however, does not control the number of conductive particles or their arrangement on the electrodes. When the number or arrangement of the conductive particles on the electrode varies, contact resistance may become inconsistent from electrode to electrode.
To reduce inconsistent contact resistance, it is possible to increase the number of conductive particles. However, this tends to increase material costs because extra conductive particles may need to be applied to the surface. Many of these are removed and discarded in the production process, resulting in waste of material.
The paper "Bare-Chip Face-down Bonding Technology Using Conductive Particles and Light-Setting Adhesives" by YAMAMURA, HYBRIDS VOL. NO. 6, pp 3-8(1992), describes a method in which an electrically insulating, light-setting adhesive is sprayed on a substrate (wafer). The electrode pad areas are masked by a photomask, which is transparent in areas that do not correspond to electrode pad areas but is opaque in areas corresponding to electrode pad areas. The light-setting adhesive is cured by irradiating it with ultra-violet light through the transparent parts of the photomask. The cured areas between the electrode pads solidifies and is not sticky while the uncured areas on the electrode pads remain sticky. Electrically conductive particles are then placed on the substrate and selectively adhere to the uncured areas that corresponds to electrode pads. The electrically conductive particles are thus selectively applied only to the electrodes of the substrate.
The methods described above may result in an arrangement of particles as shown in FIG. 13. Electrically conductive particles 52 adhere to the electrode 51 in a configuration where two or more particles 52 are stacked or staggered in the vertical direction. As a result, electrode thickness may not be even. Strict control of the spraying of the adhesive 53 is necessary to reduce or avoid this problem.
FIG. 14 shows electrically conductive particles 63 arranged over a broad surface of an electrode 62 of an electronic part 61. In this arrangement, contact may be lost in the center of the electrode 62 of electronic part 61 and an electrode 66 on substrate 65, because the force supporting the conductive particles 63 between the electrode 62 of the electronic part 61 and the electrode 66 of the substrate 65 depends on forces exerted by the adhesive 64 surrounding the patterned area of the electrode 62. The result may be unstable contact resistance.
Japanese Laid-open Patent Publication No. 3-289070/1991 discusses another connecting method in which an electrode terminal on a first electrical circuit electrically connects with an electrode terminal on a second electric circuit via conductive particles. The electrodes are mutually fixed by adhesive. In that method an adhesive layer is formed on the electrode terminal on the surface of the electrical circuit substrate. The conductive particles adhere to the adhesive layer. Thereafter, the first electrical circuit electrically connects with the electrode terminal on the second electric circuit via the conducting particles. However, when the conductive particles are put onto the insulating adhesive to connect an electronic part having a fine-pitched arrangement of electrodes, the high density of conductive particles required to make adequate contact sometimes results in short circuits between adjacent electrodes because some conductive particles may adhere to areas between the adjacent electrodes.